Near field RF (radio frequency) communication requires an antenna of one near field RF communicator to be present within the alternating magnetic field (H field) generated by the antenna of another near field RF communicator by transmission of an RF signal (for example a 13.56 Mega Hertz signal) to enable the magnetic field (H field) of the RF signal to be inductively coupled between the communicators. The RF signal may be modulated to enable communication of control and/or other data. Ranges of up to several centimeters (generally a maximum of 1 meter) are common for near field RF communicators.
Near field communication may be referred to as near-field RFID (Radio Frequency Identification) or near-field communication. NFC communicators are a type of near field RF communicator that is capable of both initiating a near field RF communication (through transmission or generation of an alternating magnetic field) with another near field RF communicator and of responding to initiation of a near field RF communication by another near field RF communicator. The term “near field RF communicator” includes not only NFC communicators but also initiating near field RF communicators such as RFID transceivers or readers that are capable of initiating a near field RF communication but not responding to initiation of a near field RF communication by another near field RF communicator and responding near field RF communicators such as RFID transponders or tags that are capable of responding to initiation of a near field RF communication by another near field RF communicator but not of initiating a near field RF communication with another near field RF communicator. Hence NFC communicators can act as both RFID transceivers and RFID transponders and are able to communicate with other NFC communicators, RFID transceivers and RFID transponders.
Examples of near field RF communicators are defined in various standards for example ISO/IEC 18092, ISO/IEC 14443, ISO/IEC 15693 ISO/IEC 21481.
Near field RF communicators may be provided as standalone or discrete devices or may be incorporated within or coupled to larger electrical devices or host devices (referred to below as near field RF communications-enabled devices) to enable those devices to communicate by the near field with other near field RF communicators or devices incorporating or coupled to such near field RF communicators. When incorporated within a larger device or host, a near field RF communicator may be a discrete entity or may be provided by functionality within the larger device or host. Examples of such larger devices or host devices are, for example, mobile telephones, portable computing devices (such as personal digital assistants, notebooks, lap-tops), other computing devices such as personal or desk top computers, computer peripherals such as printers, or other electrical devices such as portable audio and/or video players such as MP3 players, IPODs®, CD players, DVD players.
When a first near field RF communicator receives a modulated RF signal from a second near field RF communicator, this modulated RF signal is received by the antenna circuit of the first near field RF communicator and must then be demodulated by a demodulator within the near field RF communicator. However with existing designs the demodulator inputs tend to be high impedance and this can lead to a significant reduction in the Q of the antenna circuit and inability to maximize the modulated carrier signal being input to the demodulator. Where high input impedance voltage couplings are made to the demodulation circuitry, it may be necessary to divide-down the voltage of a received modulated carrier signal to avoid over-voltage damage which may otherwise occur when very high magnetic fields couple to the antenna coil. This limitation on the maximum voltage which can be allowed to develop at the antenna coil makes it difficult to adjust the dynamic range of the circuit as regards received signals.